The present invention relates to the technical field of radio frequency circuits, and in particular to a planar balun and a multi-layer circuit board on which the planar balun is formed.
In a radio frequency power amplifier, a push-pull circuit may generally be used, such as a power amplifier in which two transistors operate in a push-pull manner. In the push-pull circuit, a balun (balanced-unbalanced converter) is an essential component. For example, it can convert, at an input stage, a signal on an unbalanced transmission line into two balanced inputs with the same power but opposite phases to provide to the transistors of the power amplifier, and convert, at an output stage, two balanced outputs of the power amplifier into an unbalanced (single end) output in an opposite manner so as to provide the unbalanced output to an unbalanced load.
FIG. 1 is a schematic circuit diagram of a typical push-pull circuit. In the circuit, an input balun 1 converts a signal on an unbalanced transmission line into a balanced input of a power amplifier, and an output balun 2 converts a balanced output of the power amplifier onto an unbalanced load. Each balun in FIG. 1 is composed of two inductors, wherein the ends of one inductor serve as balanced ends of the balun and are connected to a matching network, while one end of the other inductor is grounded and the other end of the other inductor serves as an unbalanced end of the balun. By virtue of the coupling effect between the two inductors of the balun, a differential signal input from the balanced ends can be converted into a single end signal of the unbalanced end, and vice versa.
As an example, FIG. 1 also shows optional resonating capacitors respectively connected to the two inductors of each balun in parallel. By appropriately selecting the induction values of the inductors and the capacitance values of the resonating capacitors of the balun, the operational frequency of the balun can be regulated. Note that the resonating capacitors in FIG. 1 are optional elements, and if the values of the inductors are large enough, or a ¼ wavelength transmission line or a magnetic core type of inductors are used to implement the balun, the resonating capacitors can be omitted.
In the circuit in FIG. 1, a relevant circuit to provide a direct current power supply for transistors of the power amplifier is also shown. As shown in FIG. 1, gate-source voltages Vgs1 and Vgs2 can be respectively applied to gates of the two transistors via choke inductors and decoupling capacitors, and drain voltages Vdd1 and Vdd2 can be respectively applied to drains of the two transistors via choke inductors and decoupling capacitors.
Depending on different operational frequencies, the balun, such as the one shown in FIG. 1, can be implemented in different ways. For S band and above, the balun can be easily implemented with a ¼ wavelength transmission line. With regard to UHF (300 MHz to 3000 MHz), VHF (30 MHz to 300 MHz) and lower bands, as it is limited by the size and extremely high power, generally a ferrite material is used to implement the balun, since it has a higher magnetic flux density.
In a push-pull amplifier of an MR (magnetic resonance) detection system, due to a low operational frequency (tens to hundreds MHz) and high power, a balun made of ferrite material is generally used. However, since the ferrite material may form magnetic saturation, which results in that such structure cannot be used in a strong magnetic environment of a scan room of the MR detection system, it must be placed outside the scan room and connected to the MR detection system via cable. This results in a high transmission loss, and also goes against to system integration.
To this end, there is a need for a balun which is suitable to be used in strong magnetic field environment, such as a scan room of an MR detection system.